1. Field of the Invention
The present invention relates generally to UHF band transistor mixer circuits. More particularly, the present invention relates to UHF band transistor mixer circuit (referred to as "mixer circuit" hereinafter) comprising an input matching circuit of a high-pass filter type having its input portion connected through a coupling capacitor to an input terminal which is applied with a received high-frequency signal and a local oscillation signal, a frequency converting transistor having its base connected to an output portion of the above input matching circuit, and a low-pass filter having its input portion connected to a collector of the frequency converting transistor.
2. Description of the Prior Art
Mixer circuits of this type heretofore known include one shown in FIG. 3. In FIG. 3, the reference character IN denotes an input terminal which is applied with a received high-frequency signal having a frequency of fa (for example, 800 MHz) and a local oscillation signal having a frequency of fb (for example, 890 MHz) from a high-frequency amplifier (not shown) and a local oscillation circuit (not shown), respectively, the reference character HF denotes an input matching circuit of a high-pass filter type having its input portion connected to the input terminal IN through a coupling capacitor C1, and the reference character TR denotes a frequency converting transistor having its base connected to an output portion of the input matching circuit HF.
The input matching circuit HF comprises a strip-line inductor L1 inserted in series between the coupling capacitor C1 and the base of the frequency converting transistor TR, and a strip-line inductor L2 and a capacitor C2 for correcting input matching which are connected in series with each other to be inserted between the coupling capacitor C1 and a ground portion. The reference characters R1 and C3 respectively denote an emitter resistor and an alternating-current bypass capacitor which are connected in parallel with each other to be inserted between an emitter of the frequency converting transistor TR and the ground portion.
The reference character LF denotes a type low-pass filter LF which comprises a coil L3 inserted in series between the collector of the frequency converting transistor TR and an output terminal OUT and capacitors C4 and C5 connected between respective ends of the coil L3 and the ground portion.
The reference characters R2 and R3 denote resistors for dividing a direct-current voltage from a direct-current power source +B, the reference character L4 denotes a choke coil for cutting an output of the low-pass filter LF, and the reference character C6 denotes a bypass capacitor for cutting ripple components or the like of the direct-current power source +B.
In a conventional mixer circuit having such construction, the received high-frequency signal and the local oscillation signal which are inputted from the input terminal IN are matched in the input matching circuit HF and then, injected into the base of the frequency converting transistor TR. In this frequency converting transistor TR, both the above signals are frequency-converted into signals having two frequencies of (fa+fb) and (fa-fb) (i.e., 800 MHz+890 MHz=1690 MHz and 890 MHz-800 MHz=90 MHz) by utilizing a non-linear active region thereof. Only the signal of (fa-fb) which is a desired frequency of two frequency-converted outputs, i.e., a signal component of 90 MHz (referred to as "desired signal component" hereinafter) is outputted from the output terminal OUT via the low-pass filter LF as an intermediate frequency signal to be inputted to an intermediate frequency amplifier (not shown).
In addition, in such a mixer circuit, the input matching circuit HF is formed as a high-pass filter type so as to prevent the signal of (fa-fb) which is the output of the low-pass filter LF, i.e., the desired signal component of 90 MHz from being returned to the base of the frequency converting transistor TR from the input terminal IN through the input matching circuit HF, that is, prevent the desired signal component from being reflected at the input terminal IN and inputted to the base of the frequency converting transistor TR.
FIG. 4 is a diagram of return loss characteristics showing the return loss relative to the frequency in the input matching circuit HF. This return loss can be derived by measuring the absolute value of the impedance in a high-frequency circuit, which shows to what extent a reflected wave is attenuated with respect to an incident wave. According to the return loss characteristics, the return loss becomes approximately zero relative to a frequency of 750 MHz or less, particularly the desired signal component of 90 MHz. Accordingly, the desired signal component is not reflected from the input terminal IN, and therefore, the same is not to be returned to the base of the frequency converting transistor TR.
Furthermore, the return loss becomes approximately 8 to 10 dB relative to a frequency region of 750 MHz or more, i.e., a received high-frequency signal of 800 MHz and a local oscillation signal of 890 MHz. Accordingly, both the signals are efficiently transmitted and injected to the base of the frequency converting transistor TR through the input terminal IN.
As described above, the input matching circuit HF is formed as a high-pass filter type so as to prevent the return of the desired signal component of 90 MHz. In the conventional example, however, unnecessary signals of 750 MHz or more outside the respective frequency regions of the received high-frequency signal and the local oscillating signal are passed over a wide band, as shown in FIG. 4. Accordingly, input matching characteristics in the input matching circuit HF is somewhat inferior, thereby encountering a problem of adversely affecting characteristics such as the noise figure, conversion gain and intermodulation in the mixer circuit.